Method and apparatus of a fast two-loop automatic gain control circuit

ABSTRACT

In a burst-mode, high-speed spread-spectrum communications system, faster convergence of a receiver&#39;s automatic gain control (AGC) circuit reduces the time required to bring a received signal within the operating range of an operation amplifier and other radio-frequency and digital sections of the receiving system. A gain control circuit includes a coarse-gain feedback loop and a fine-gain feedback loop to improve convergence speed and at the same time maintain the stability of the AGC circuit. The coarse-gain feedback loop quickly brings the received signal, using a large gain signal, to the desired operating range. The fine-gain feedback loop uses a smaller gain signal to gradually smooth the received signal to avoid saturation on the A/D converters.

FIELD OF THE INVENTION

This invention is related to digital communications systems, and moreparticularly to a programmable, fast response and fast convergenceautomatic gain control (AGC) circuit for packet communication systems.

BACKGROUND

Mobile communication is becoming increasingly popular and the recentadvances in digital processing have allowed a rapid migration of mobileservices from analog communications to digital communications.Increasingly, development efforts are focusing on techniques forhigh-capacity communication of digital information over wireless links,and much of this wireless development work incorporates spread-spectrumcommunications technology.

Spread-spectrum is a method of modulation that spreads a data signal fortransmission over a bandwidth, which substantially exceeds the datatransfer rate. Direct sequence spread-spectrum involves modulating adata signal onto a pseudo-random chip sequence. The chip sequence is thespreading code sequence for spreading the data over a broad band of thespectrum. The spread-spectrum signal is transmitted as a radio wave overa communications media to a receiver. The receiver despreads the signalto recover the information data. The evolution of wirelesscommunications to very high data rates with packet transmissions overthe air has imposed constraints on the receiving system radio-frequency(RF) stages as well as on the operation of the analog-to-digitalconverters used therein.

Due to large variations in received signal power caused by propagationattenuation (e.g., fading due to man-made objects such as buildings ornatural terrain features such as hills), a control mechanism is oftenused to dynamically control the gain of the receiving amplifier so thatsubsequent radio-frequency (RF) sections and digital sections of thereceiving system can operate within a desired operating range. Thesesections include amplifiers, mixers, analog-to-digital converters andbaseband analog or digital processing devices. The control mechanism foradjusting the amplification of the received signal level is referred toas automatic gain control (AGC).

An AGC circuit is designed to keep the amplified received signal at anear-constant power level over a large range of received signal levels.Three parameters involved in designing an AGC circuit include itsoperational range, its response time, and its steady-state error. Theoperational range of an AGC circuit in current spread-spectrumcommunications systems can easily exceed 80 to 90 dB in signal power.Some conditions that can contribute to this wide operating range includesignal attenuation caused by hills or buildings and power controlfailure occurring when a mobile transmitter is in close proximity to abase station receiver.

Normally in dynamic control systems, the response time of the system isinversely related to its steady state error. As the control systemparameters are configured and set to improve the system's response time,system instability, such as overshoot conditions, are likely toincrease. In high data rate digital communications, and especially inpacket switched systems, the conflict between these last two designparameters becomes increasingly important. In these types of systems,the data transmission interval can be as small as a fraction of amillisecond and even shorter. Because the start of each packetintroduces a large signal variation and the periods of symbols within apacket are so short, a conventional AGC is not able to ensure timelyamplification control for the received signal and, therefore, will proveineffective at providing reliable data communications. Under thesecircumstances, a need, unmet by conventional AGC circuitry, exists foran AGC circuit that can quickly adjust the gain of a received signalduring only a small period of time after a large received signal powerfluctuation and can also provide smooth and stabilized operation duringthe remainder of data reception.

SUMMARY OF THE INVENTION

The present invention provides a fast AGC circuit through the use of afine-gain feedback loop and a coarse-gain feedback loop to provide boththe speed and control needed for high-speed digital communicationsystems. In addition, by utilizing circuits already present in a typicalreceiver and avoiding any redundant introduction of similar circuitry ineach individual control loop, the inventive AGC circuit reduces both thesize and cost of the hardware when compared to earlier systems.

Aspects of the present invention relate to a method for automaticallyvarying a gain control signal in a receiver amplifier that has anadjustable gain value based on the gain control signal. In this method,an amplified received signal is quantized, and its power is measured.Next, the measured power is compared to a reference power to calculatean error signal and the magnitude of the error signal is compared to athreshold value to determine whether to use a fine-gain feedback loop ora coarse-gain feedback loop. If the magnitude of the error signal isgreater than the threshold, then a loop with a large gain is selected,to speed-up the convergence. If the magnitude of the error signal isless than or equal to the threshold value, then a loop with a smallergain is selected, to prevent saturation.

Other aspects of the present invention relate to an automatic gaincontrol circuit for a receiver. The receiver includes a voltagecontrolled amplifier (VCA) that amplifies a received signal according toa gain control signal. A power meter measures the power of the amplifiedsignal. A fine-gain loop multiplies the error signal by a fine-gainconstant, and a coarse-gain loop multiplies the error signal by acoarse-gain constant. The values of the gain constants can be determinedby experiments to suit a specific system operating environment. Afeedback filter receives only one of the amplified error signals andprovides it to the VCA amplifier for controlling its gain. Theappropriate feedback loop signal is selected by a signal selector basedon whether the magnitude of the error signal is less than or equal tothe threshold value (the fine-gain feedback loop is selected) or isgreater than the threshold value (the coarse-gain feedback loop isselected) to provide the gain control signal to the VCA amplifier forcontrolling its gain.

Further aspects of the present invention relate to a spread-spectrumtransceiver having AGC circuitry and a method for including automaticgain control in a spread-spectrum receiver. The inventive devices andmethods include a two-loop AGC circuit, as described above, havingcoarse and fine gain control that are selectively activated depending onwhether the magnitude of the difference between the amplified receivedsignal power and a reference power exceeds a predetermined thresholdvalue.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing and the accompanying drawings or may be learned by practice ofthe invention. The objects and advantages of the invention may berealized and attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawing figures depict preferred embodiments of the presentinvention by way of example, not by way of limitations. In the figures,like reference numerals refer to the same or similar elements.

FIG. 1 illustrates a schematic of an automatic gain control circuitaccording to an embodiment of the present invention.

FIGS. 2A, 3A illustrate exemplary signal levels received byautomatically gain controlled circuitry utilizing embodiments of thepresent invention.

FIGS. 2B and 3B illustrate exemplary signal levels received beforeapplying the automatically gain controlled circuitry utilizingembodiments of the present invention.

FIG. 4 illustrates an exemplary spread-spectrum transmitter in whichembodiments of the present invention have application.

FIG. 5 illustrates an exemplary spread-spectrum receiver in whichembodiments of the present invention have application.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 illustrates an automatic gain control circuit 100 according tocertain embodiments of the present invention for use with a receiversystem or a transceiver system. While the AGC circuit 100 is describedwithin the exemplary environment of a direct sequence spread-spectrumreceiver, the present AGC circuit also has application to other types ofreceiver systems that would similarly benefit from fast, dynamic controlof the amplification of the received signal. The present invention hasbenefit within a receiver of both base stations and mobile stations.

Previous efforts undertaken have investigated fast response AGCcircuitry for high-speed communications systems. These previous efforts,however, involved generating coarse signal strength information from theanalog received signal and producing finer control information fromseparate analog-to-digital converted signal inputs. The coarse and fineinformation were then used in combination with a look-up table togenerate gain control signals for the receiver's front-end VCAamplifier.

The present AGC circuit 100, however, includes a fine-gain feedback loopand a coarse-gain feedback loop but, unlike previous attempts atproviding two-loop controlled systems, the AGC circuit 100 utilizescommon elements, such as A-to-D converters and filters, for both loops.Accordingly, the AGC circuit 100 provides two gain control loops, thatimprove AGC convergence speed without the increased size and cost thatresult from unnecessarily redundant circuitry.

In a preferred embodiment illustrated in FIG. 1, a multi-channelspread-spectrum signal is received at a receive antenna 15 and isamplified by a voltage controlled amplifier 16. In a spread-spectrumreceiving system, the amplified signal is provided to frequencytranslators 17 and 19 which are supplied with in-phase and out-of-phasesignal component of the local carrier by a voltage controlled oscillator(VCO) 18. As a result, a down-converted, multi-channel in-phase andout-of-phase baseband signal is forwarded to a respective one of the A/Dconverters 20 for analog-to-digital conversion. The converted in-phasesignal is received at a matched filter 22 and the converted out-of-phasesignal is received at a similar matched filter 23, where the signals areprocessed (discussed later with regard to FIG. 5).

The voltage controlled amplifier 16 has a variable gain that isadjustable according to a control signal. The remaining AGC circuitelements 2444 are used to generate this control signal at an appropriatevalue.

The quantized signals from the A/D converters 20 are measured by powermeters 24 and 25 averaged over a fraction of the spreading-symbol periodand then summed as the received signal strength indicator (RSSI). Inparticular, the converted in-phase signal power is measured by meter 24and the converted out-of-phase signal power is measured by meter 25. Thepower level of each of the quantized signals at a particular time ismeasured by squaring the respective signals and averaged over a fractionof the spread-symbol period. These measured powers from each signal arethen summed by the adder 26 to determine the RSSI signal.

The RSSI signal is preferably filtered by a low-pass filter 27 whichoperates to smooth any abrupt signal variations of the RSSI signal. Thesmoothed RSSI signal is then limited by the gain limiter circuit 28. Thegain limiter 28 includes a pre-set maximum RSSI value, RSSI_(max), and apre-set minimum RSSI value, RSSI_(min). These constraining values permitthe AGC circuit 100 to control the range of the RSSI signal that isforwarded on to the later circuit stages. For example, because theupcoming log( ) function has a singular point at 0, RSSI_(min) can beset to some positive number to avoid the singularity. The pre-set valuesof RSSI_(max) and RSSI_(min) are adjustable depending on differentreceiver characteristics such as the desired AGC convergence speed andthe required A/D resolution.

Next, the constrained RSSI signal is compressed by block 29 whichperforms the logarithm function on the signal. Compressing the RSSIsignal by taking the logo is beneficial, because doing so reduces therange of values involved in the subsequent linear processing steps ofthe AGC control signal.

An error signal is then generated by subtracting the compressed RSSIsignal from the preferred power level (i.e. a reference level 30) viasubtractor 31. The magnitude of the resulting error signal provides anindication of the difference between the actual power level of thesignal currently being received and a desired power level.

In the preferred embodiment, the error signal is then forwarded to afine-gain and a coarse-gain multiplier. Specifically, the error signalis amplified by the fine-gain constant 33 and the coarse-gain constant34. Along with the feedback filter 41, the coarse-gain constant 34provides a feedback loop with a large gain control signal that quicklybrings the multi-channel spread-spectrum signal near the operating rangeof the amplifier 16. Along with the feedback filter 41, the fine-gainconstant 33 provides a feedback loop with a smaller gain control signalthat gradually smoothes the received signal to the operating range ofthe A/D converters 20 while avoiding saturation.

The feedback filter 41 is a conventional feedback filter operating on asampling clock. At each clock, the feedback filter adds at 35 thecurrent output from either the coarse-gain amplified error signal at theoutput of multiplier 34 or the fine-gain amplified error signal at theoutput of the multiplier 33 to its previous output (i.e., the output ofthe filter 41 delayed by one clock time through delay element 36). Thisfeedback filter operates to smooth the amplified error signal.

Selector circuit 43, which is an electronically controlled switch,selectively controls whether it is the fine-gain feedback loop signal orthe coarse-gain feedback loop signal that is forwarded to control thegain of the amplifier 16. The control of the selector 43 for switchingbetween the fine-gain and coarse-gain feedback loops is automaticallycontrolled by a predetermined error threshold 44 that is adjustable tosuit each individual receiver application.

The magnitude of the error signal from the subtractor 31 is measured bythe absolute value lock 32. This error signal magnitude is thencompared, via the comparator 42, to an error threshold value 44. Theselective operation of selector circuit 43 varies according to therelationship between the error signal magnitude and the threshold value44. As depicted in FIG. 1, if the magnitude of the error signal exceedsthe threshold 44, then the selector 43 switches to the coarse-gainfeedback loop; otherwise, the selector 43 switches to the fine-gainfeedback loop. In operation, the fine-gain feedback loop is selectedwhen the magnitude of the error signal and the threshold value 44satisfy a first relationship (i.e., the magnitude of the error signal isless than or equal to the threshold) and the coarse-gain feedback loopis selected when the complimentary relationship between the two valuesis satisfied (i.e., the magnitude of the error signal is greater thanthe threshold).

Initially, when there is a large variation of signal power over theoperating range of the receiver amplifier, the large gain factor in thecoarse-gain feedback loop is applied to the received signal to bring thepower level to the desired range. After that, a small gain factor isapplied in the fine-gain feedback loop to smooth out some of thevariations which might cause the A/D converter to saturate. The errorthreshold 44 determines when the AGC circuit switches to the fine-gainfeedback loop from the coarse-gain feedback loop, and vice-versa.Furthermore, the fine-gain feedback loop with a small gain controlsignal stabilizes the feedback loop when the steady-state error signalis very small, such as when the AGC circuit actually reaches thesteady-state.

The gain control signal, resulting from either the fine-gain orcoarse-gain feedback loops, is then re-converted to a linear power byapplying the exponential function, via circuitry 37, to the gain signal.The gain signal is then converted to an analog signal by the D/Aconverter 38 and smoothed by an RC low pass filter 39 to generate anamplifier gain control signal. This control signal adjusts the gain ofthe amplifier 16.

FIGS. 2A and 2B illustrate an AGC-controlled multi-channelspread-spectrum signal at a receiver utilizing an embodiment of thepresent invention. Each figure, which shows the amplified receivedsignal corresponding to four different packets, depicts the in-phasesignal on top and the out-of-phase signal on the bottom. As seen fromFIG. 2A, the AGC-controlled signal has a set of spikes in the beginningof the packet, with this group of spikes having a duration of roughlyone symbol. After such a short duration, the AGC-controlled signalquickly settles between the range of −1.5 volts to +1.5 volts withoutexperiencing saturation. For comparison, conventional AGC circuitstypically provide convergence times of between 40 and 50 symbol periods.FIG. 2B shows the received spread-spectrum signal which has beensubjected to a 46 dB attenuation under Rayleigh fading.

FIG. 3A illustrates the AGC-controlled multi-channel spread-spectrumsignal when the received signal, shown in FIG. 3B, has been subjected toa 46 dB power surge under Rayleigh fading. Thus, in a Rayleigh fadingenvironment, FIGS. 2A, 2B, 3A and 3B illustrate the dynamic range of thepresent inventive AGC circuit to be 92 dB.

The inventive AGC circuitry is particularly useful in a burst-mode,high-speed spread spectrum receiver. Accordingly, it may be helpful atthis point to summarize the structure and operation of an exemplaryspread spectrum communication system in general and the receiver in sucha system in particular. FIG. 4 and FIG. 5 depict a spread-spectrumcommunications system transmitter and receiver respectively forproviding high-speed service.

The transmitter section essentially includes the elements 111-134 shownFIG. 5. An encoder 111 receives input information data, for example at28 Mbps. The encoder 111 performs error correction encoding, for exampleby application of a rate-1/2 convolutional code. The resultant encodeddata at 56 Mbps is applied to an interleaver 112. At the output of theinterleaver 112, the data stream is divided into a number of sub-channeldata streams, by a demultiplexer 113. In this example, the data streamis split into n branches, d₁(t) to d_(n)(t).

Each sub-channel data sequence goes to an input of two mapper circuitse.g. the first sub-channel d₁(t) goes to phase map 114 and code map 115.Each code map maps m bits of the sub-channel data sequence to a distinctone of the available code-spreading sequences. Each phase map maps kbits of the sub-channel data sequence to a distinct one of the availablecomplex phasors. A multiplier 116 as in the first sub-channel modulatesthe chosen phasor on the mapped spreading sequence over the sequencesymbol period. In a similar manner, the product device 119 multiplieseach code sequence selected by the code map circuit 118 by the phasorselected by the phase map circuit 117, to form the spread spectrumsignal for the second sub-channel. Similar devices perform the samefunctions for the other sub-channels; and so the product device 122multiplies code sequences from selected by the code map circuit 121 bythe corresponding phasors selected by the map circuit 120, to form thesignal for the n-th sub-channel.

The complex signal combiner 123 algebraically combines the real orin-phase components of the spread-spectrum sub-channels from the productdevices, to form an in-phase (I) multi-channel spread-spectrum signal.The complex signal combiner 123 algebraically combines the imaginary orquadrature components to form a quadrature (Q) multi-channelspread-spectrum signal. The resultant multi-channel spread-spectrumin-phase signal and quadrature signal is spread by a cell-site specificsignature sequence 125 and 127 respectively, such as an extended Goldsequence (g). Multipliers 128, 130 modulate carrier-frequency signals129, 131 generated by a local oscillator to shift the in-phase andquadrature signal to a radio frequency respectively. The transmitteralso includes a summing device 132 and a power amplifier 133 fortransmitting the combined signal over a communications channel via anantenna 134.

The receiver shown in FIG. 5 essentially comprises the elements 141-153.The receiver includes an antenna 141 for receiving the spread-spectrumsignal transmitted over the air-link. A VCA amplifier 142 controlled bythe inventive AGC circuit through the gain control signal provides lownoise amplification of the analog signal from the antenna 141. Forconvenience, the AGC control loops have been omitted here. The AGC,however, would be implemented as shown, for example, in FIG. 1.

The VCA amplifier 142 supplies the channel signal to two translatingdevices 143 and 144. A local oscillator generates propercarrier-frequency signals and supplies a cos(ω₀t) signal to the device143 and supplies a sin(ω₀t) signal to the device 144. The translatingdevice 143 multiplies the amplified over-the-air channel signal by thecos(ω₀t) signal; and the translating device 144 multiplies the amplifiedover-the-air channel signal by the sin(ω₀t) signal. The translatingdevices 143 and 144 translate the received multi-channel spread-spectrumsignal from the carrier frequency to the baseband.

The translating device 143 downconverts the spread-spectrum signal tothe baseband and supplies the converted signal to an analog to digital(A/D) converter 145. Similarly, the translating device 144 downconvertsthe spread-spectrum signal to the baseband and supplies the convertedsignal to an analog to digital (A/D) converter 146. Each of the digitaloutput signals is applied to a matched filter (MF) bank 147 or 148. Eachmatched filter bank 147, 148 utilizes a matrix of potential spreadingcodes as reference signals, for example to detect any one or more of allpossible spreading codes, and correlate the signal on its input toidentify the most likely matches. In this manner, each MF bank 147 or148 selects the most probably transmitted code sequence for therespective channel.

The signals from the MF banks 147 and 148 are supplied in parallel to aprocessor 149, which performs automatic frequency correction (AFC) andphase rotation, and the outputs thereof are processed through a Rakecombiner and decision/demapper circuit 11, to recover and remap the chipsequence signals and phasor signals to the original data sequences. Thedata sequences for the I and Q channels also are multiplexed together toform a data stream at 56 Mbps. This detected data stream is applied to adeinterleaver 152. The deinterleaver 152 reverses the interleavingperformed by element 112 at the transmitter. A decoder 153 performsforward error correction on the stream output from the deinterleaver152, to correct errors caused by the communication over the air-link andthus recover the original input data stream (at 28 Mbps).

The illustrated receiver also includes a clock recovery circuit 154, forcontrolling certain timing operations of the receiver, particularly theA/D conversions.

The present invention provides an improvement in the circuitry thatgenerates the AGC gain control signal input to the VCA amplifier 142, asshown in the receiver of FIG. 5. In accord with the invention, the AGCgain control signal is provided by circuitry using two feedback loopsthat provide fast convergence and stable operation, as discussed indetail above, with respect to FIG. 1.

In the above-discussed example of FIG. 4 and FIG. 5, for simplicity,there was essentially one transmitter sending signal to one receiver.The actual application of the invention may involve multi-casting tomultiple receivers. A preferred network, however, would include a numberof cell-site base stations connected to a broadband packet network (notshown).

Each base station would include a transmitter and a receiver utilizingcell-site specific cover codes. A number of mobile stations wouldcommunicate with each base station. Within each cell, the mobilestations would access the base station in a time division manner. In asimilar fashion, the cell site base station would transmit to eachmobile station on a time division basis. In any two-way communicationnetwork, all stations would include a transmitter and a receiver. Forexample, both the base stations and the mobile stations in the cellularnetwork would include a transmitter and a receiver, such as disclosedwith regard to FIGS. 4 and 5. In both directions, the receiver wouldutilize one or more matched filters implemented in accord with thepresent invention.

Those skilled in the art will recognize that the spread-spectrumcommunication system of FIGS. 4 and 5 is but one example of aspread-spectrum system that may utilize a fast convergence AGC circuit,in accord with the invention. For example, the inventive AGC circuitryis similarly applicable to CDMA cellular receivers, in wideband CDMAreceivers, TDMA receivers, etc.

FIG. 1 illustrates a preferred arrangement of the comparator and controlcircuitry for implementing selectable coarse and fine feedback loops.One of ordinary skill would easily recognize that alternative circuitrycan accomplish the same function without departing from the scope of thepresent invention.

While the foregoing has described what are considered to be the bestmode and/or other preferred embodiments of the invention, it isunderstood that various modifications may be made therein and that theinvention may be implemented in various forms and embodiments, and thatit may be applied in numerous applications, only some of which have beendescribed herein. It is intended by the following claims to claim anyand all modifications and variations that fall within the true scope ofthe invention.

1. A fast two-loop automatic-gain-control (AGC) method for automaticallyvarying a gain control signal for a receiver's variable amplifier,comprising the steps of: a) amplifying a received signal according to anadjustable gain value, wherein the adjustable gain value is based on again control signal; b) converting the amplified received signal fromanalog to digital c) calculating an indicator of the received signal'sstrength; d) generating an error signal based on the indicator and apredetermined reference power level; e) if a magnitude of the errorsignal bears a first relationship to a predetermined threshold value,then varying the gain control signal as a function of a fine-gainconstant and the error signal; and f) if the magnitude of the errorsignal bears a second relationship to the predetermined threshold value,then varying the gain control signal as a function of a coarse-gainconstant and the error signal; wherein the coarse-gain constant islarger than the fine-gain constant and the second relationship iscomplimentary to the first relationship.
 2. The method according toclaim 1, further comprising the steps of: g) amplifying the error signalbased on the fine-gain constant to generate a first signal; h)amplifying the error signal based on the coarse-gain constant togenerate a second signal; and wherein if the magnitude of the errorsignal bears the first relationship to the predetermined thresholdvalue, then varying the gain control signal in proportion to the firstsignal; and if the magnitude of the error signal bears the secondrelationship to the predetermined threshold value, then varying the gaincontrol signal in proportion to the second signal.
 3. The methodaccording to claim 2, wherein: the step of varying the gain controlsignal in proportion to the first signal includes the steps of: if themagnitude of the error signal bears the first relationship to thepredetermined threshold value, connecting the first signal, if notalready connected, to an input of a feedback filter and disconnectingthe second signal, if already connected, from the input of the feedbackfilter, wherein the feedback filter includes an output that varies inproportion to the input, and varying the gain control signal accordingto the output of the feedback filter; and the step of varying the gaincontrol signal in proportion to the second signal includes the steps of:if the magnitude of the error signal bears the second relationship tothe predetermined threshold value, connecting the second signal, if notalready connected, to the input of the feedback filter and disconnectingthe first signal, if already connected, from the input of the feedbackfilter, and varying the gain control signal according to the output ofthe feedback filter.
 4. The method according to claim 3, wherein thefeedback filter comprises: a delay circuit configured to delay theoutput of the feedback filter one clock period; and a signal combinerconfigured to add a current input to the feedback filter to the delayedoutput.
 5. The method according to claim 1, wherein the step ofcalculating the indicator includes the steps of: c.1) generating anin-phase signal based on the amplified received signal; c.2) convertingthe in-phase signal from analog to digital; c.3) generating anout-of-phase signal based on the amplified received signal; c.4)converting the out-of-phase signal from analog to digital; c.5)calculating an in-phase signal power, wherein the in-phase signal poweris substantially equal to the square of the converted in-phase signalaveraged over a fraction of a spreading-symbol period; c.6) calculatingan out-of-phase signal power, wherein the out-of-phase signal power issubstantially equal to the square of the converted out-of-phase signalaveraged over the fraction of the spreading-symbol period; and c.7)summing the in-phase signal power and the out-of-phase signal power toobtain the indicator.
 6. The method according to claim 1, furthercomprising the steps of: g) low pass filtering the indicator; h)limiting the filtered indicator to be between a predetermined minimumvalue and a predetermined maximum value; i) calculating a logarithm ofthe limited indicator; and wherein generating of the error signal isbased on the calculated logarithm of the limited indicator.
 7. Themethod according to claim 1, further comprising the steps of: g)calculating the exponential of the gain control signal; h) convertingthe calculated exponential of the gain control signal from digital toanalog; and i) low pass filtering the converted gain control signal;wherein the adjustable gain value of the variable amplifier is based onthe low-pass-filtered, converted gain control signal.
 8. The methodaccording to claim 6, wherein the step of generating an error signalincludes the steps of: d.1) storing the predetermined reference powerlevel; and d.2) subtracting the logarithm of the limited indicator fromthe predetermined reference power level.
 9. The method according toclaim 1, wherein the first relationship corresponds to the magnitude ofthe error signal being less than or equal to the predetermined thresholdvalue.
 10. The method according to claim 1, wherein varying of the gaincontrol signal is based on an amplified error signal that is generatedby selectively connecting the error signal with only one of two signalmultipliers to a feedback filter according to a magnitude of the errorsignal, wherein if the magnitude of the error signal bears the firstrelationship to the predetermined threshold value, the error signal isconnected to a multiplier having the fine-gain constant outputting tothe feedback filter and if the magnitude of the error signal bears thesecond relationship to the predetermined threshold value, the errorsignal is connected to a second multiplier having the coarse-gainconstant outputting to the feedback filter.
 11. The method according toclaim 10, wherein varying of the gain control signal is based on theamplified error signal after being filtered by: delaying the gaincontrol signal; and combining the amplified error signal with thedelayed gain control signal.
 12. A fast two-loop automatic gain controlcircuit for a receiver, comprising: a receiver amplifier having at leasta received signal and a gain control signal as separate inputs, whereinthe receiver amplifier amplifies the received signal in proportion to anadjustable gain value controlled by the gain control signal; a powermeter configured to measure a received signal strength indicatorassociated with the amplified received signal; a first signal combinerconfigured to generate an error signal, wherein the error signal is afunction of a reference power level and the measured received signalstrength indicator; a feedback filter having as a first output the gaincontrol signal input of the receiver amplifier; a fine-gain loop,configured to receive, as input, the error signal and further configuredto generate, a first signal for output to the feedback filter, the firstsignal being based on the error signal and a fine-gain constant; acoarse-gain loop, configured to receive, as input, the error signal andfurther configured to generate a second signal for output to thefeedback filter, the second signal being based on the error signal and acoarse-gain constant larger than the fine-gain constant; and a selectorcircuit configured to selectively cause the fine and coarse loops toselectively apply only one of the loops at a time to drive the feedbackfilter, wherein the fine-gain loop is selected when a magnitude of theerror signal bears a first relationship to a predetermined thresholdvalue such that the gain control signal varies according to the firstsignal; the coarse-gain loop is selected when the magnitude of the errorsignal bears a second relationship to the predetermined threshold valuesuch that the gain control signal varies according to the second signal;and the second relationship is complimentary to the first relationship.13. The automatic gain control circuit according to claim 12, furthercomprising: a quadrature down-converter configured to separate theamplified received signal into an in-phase signal and an out-of-phasesignal; a first analog-to-digital converter configured to convert thein-phase signal from analog to digital; a second analog-to-digitalconverter configured to convert the out-of-phase signal from analog todigital; and wherein the power meter comprises: a first power calculatorconfigured to output the square of the converted in-phase signalaveraged over a fraction of a spreading-symbol period; a second powercalculator configured to output the square of the converted out-of-phasesignal averaged over the fraction of the spreading-symbol period; and asecond signal combiner configured to sum the respective outputs from thefirst and second power calculators to obtain the received signalstrength indicator.
 14. The automatic gain control circuit according toclaim 13, further comprising: a power-shaping circuit connected betweenthe power meter and the first signal combiner, comprising: a low passfilter configured to receive the measured received signal strengthindicator from the second signal combiner; a gain limiter configured toreceive a second output from the low pass filter and limit the value ofthe second output between a predetermined minimum and maximum value, anda logarithm calculator, configured to calculate the logarithm of a thirdoutput from the gain limiter and provide the calculated logarithm to thefirst signal combiner.
 15. The automatic gain control circuit accordingto claim 12, further comprising: a magnitude calculator configured tocalculate the magnitude of the error signal; and a comparator configuredto: receive the calculated magnitude of the error signal, receive thepredetermined threshold value, and output a decision value to the signalselector, wherein the decision value indicates which of the first andthe second relationships is satisfied by the calculated magnitude of theerror signal.
 16. The automatic gain control circuit according to claim15, wherein the first relationship corresponds to the calculatedmagnitude of the error signal being less than or equal to thepredetermined threshold value.
 17. The automatic gain control circuitaccording to claim 12, wherein the feedback filter comprises: a delaycircuit configured to delay the output of the feedback filter one clockperiod; and a third signal combiner configured to add a current input tothe feedback filter to the delayed output.
 18. The automatic gaincontrol circuit according to claim 12, further comprising a controlsignal shaper connected between the output of the feedback filter andthe gain control signal input of the receiver amplifier, wherein thecontrol signal shaper comprises: an exponential calculator configured tocalculate the exponential of the output of the feedback filter; adigital to analog converter configured to receive and convert thecalculated exponential from digital to analog; and a low pass filterconfigured to: receive and filter the converted, calculated exponential,and output the filtered, converted, calculated exponential to the gaincontrol signal input of the receiver amplifier.
 19. The automatic gaincontrol circuit according to claim 12, wherein: the fine-gain loopcomprises the fine-gain amplified error signal; the coarse-gain loopcomprises the coarse-gain amplified error signal; and the selectorcircuit comprises: a first input connected with the output of thefine-gain loop; a second input connected with the output of thecoarse-gain loop; and wherein the selector circuit is configured to:output the fine-gain amplified error signal to the feedback filter onlywhen the magnitude of the error signal bears the first relationship tothe predetermined threshold value; and output the coarse-gain amplifiederror signal to the feedback filter only when the magnitude of the errorsignal bears the second relationship to the predetermined thresholdvalue.
 20. A fast two-loop AGC circuit for use in a communicationstransceiver, comprising: (a) a receiver configured to receive a firstcommunications signal and to amplify the received signal with a variableamplifier, wherein the receiver further comprises: (a)(1) a quadraturedownconverter connected with the variable amplifier configured togenerate an in-phase signal and an out-of-phase signal; (a)(2) a firstanalog-to-digital converter configured to generate a converted in-phasesignal by converting the in-phase signal from analog to digital; (a)(3)a second analog-to-digital converter to generate a convertedout-of-phase signal by converting the out-of-phase signal from analog todigital (b) a transmitter configured to transmit a second communicationssignal; and (c) an automatic gain control circuit configured to generatea control signal to vary the gain of the variable amplifier, wherein theautomatic gain control circuit further comprises: (c)(1) a first powermeter configured to measure a first power averaged over a fractionalspread-symbol period associated with the converted in-phase signal;(c)(2) a second power meter configured to measure a second poweraveraged over the fractional spread-symbol period associated with theconverted out-of-phase signal; (c)(3) a composite power meter configuredto combine the first and second power into a composite power as thereceived signal strength indicator; (c)(4) a first signal combinerconfigured to generate an error signal, wherein the error signal is afunction of both the received signal strength indicator and a referencepower level; (c)(5) a fine-gain feedback filter configured to output thecontrol signal, wherein the control signal varies proportionally to theerror signal and a fine-gain constant; (c)(6) a coarse-gain feedbackfilter configured to output the control signal, wherein the controlsignal varies proportionally to the error signal and a coarse-gainconstant, wherein the coarse-gain constant is larger than the fine-gainconstant; (c)(7) a selector circuit configured to: select only thefine-gain feedback filter when the error signal bears a firstrelationship with a predetermined threshold value; and select only thecoarse-gain feedback filter when the error signal bears a secondrelationship with the predetermined threshold value; and wherein thesecond relationship is complimentary to the first relationship.
 21. Thetransceiver according to claim 20, wherein the transceiver is located ina base station.
 22. The transceiver according to claim 20, wherein thetransceiver is located in a mobile station.
 23. The transceiveraccording to claim 20, wherein both the transmitted and received signalsare non-spread-spectrum signals.
 24. The transceiver according to claim23, wherein both the transmitted and received signals are orthogonalfrequency division multiplexing (OFDM) signals.
 25. An automatic gaincontrol circuit for a receiver, comprising: a receiver amplifier havingat least a received signal and a gain control signal as separate inputs,wherein the receiver amplifier amplifies the received signal inproportion to an adjustable gain value controlled by the gain controlsignal; a power meter configured to measure a power level associatedwith the amplified received signal; a first signal combiner configuredto generate an error signal, wherein the error signal is a function of areference signal level and the measured power level; a loopback filterfor supplying the gain control signal input to control the adjustablegain value of the receiver amplifier; a selective-gain loop, configuredto receive the error signal as input, and further configured toselectively generate first and second signals as outputs for applicationto drive the loopback filter, wherein: the first signal is based on theerror signal and a first gain constant, and the second signal is basedon the error signal and a second gain constant larger than the firstgain constant; and a control circuit coupled to control the selectiveoperation of the selective-gain loop in response to a magnitude of theerror signal, such that the selective-gain feedback loop outputs thefirst signal when the magnitude of the error signal bears a firstrelationship to a predetermined threshold value, and the selective-gainfeedback loop outputs the second signal when the magnitude of the errorsignal bears a second relationship to the predetermined threshold value;and wherein the second relationship is complimentary to the firstrelationship.
 26. An automatic gain control circuit for use with adigital receiver, the digital receiver including an amplifier having atleast a received signal and a gain control signal as separate inputs,wherein the amplifier amplifies the received signal in proportion to anadjustable gain value controlled by the gain control signal and outputsan amplified signal for application to a digital to analog converter ofthe digital receiver, said automatic gain control circuit comprising: apower meter for measuring a power level associated with the amplifiedreceived signal in response to a digitized output from the digitalanalog converter of the digital receiver; a combiner for generating anerror signal as a function of a reference signal level and the measuredpower level; a selective-gain feedback loop for supplying the gaincontrol signal input to control the adjustable gain value of thereceiver amplifier, the selective-gain feedback loop being configuredto: receive the error signal as an input, and selectively generate thegain control signal based on application of first and second gainconstants to the error signal, the second gain constant being largerthan the first gain constant; and a control circuit coupled to controlselective operation of the selective-gain feedback loop in response to amagnitude of the error signal, such that the selective-gain feedbackloop applies the first gain constant when the magnitude of the errorsignal bears a first relationship to a predetermined threshold value,and the selective-gain feedback loop applies the second gain constantwhen the magnitude of the error signal bears a second relationship tothe predetermined threshold value; wherein the second relationship iscomplimentary to the first relationship.